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Electromigration

Electromigration is an electrochemical process where metal on an insulating material, in a humid environment and under an applied electric field, leaves its initial location in ionic form and redeposits somewhere else. Such migration may reduce isolation gaps and ultimately lead to an electrical short circuit.

The process begins when a thin continuous film of water has been formed and a potential is applied between oppositely charged electrodes.

Positive metal ions are formed at the positively biased electrode1 (the anode), and migrate toward the negatively charged cathode. Over time, these ions accumulate as metallic dendrites, reducing the spacing between the electrodes, and eventually creating a metal bridge.

  1. Electromigration is closely related to corrosion, with the anode being attacked, but which circuit element is the anode is determined by the applied field rather than the oxidation potential of the metal used.

Whilst most often seen as a surface effect, vertical migration can also occur when moisture has penetrated into the bulk of a porous material.

 

Dendritic growth across tracks on a PCB

Dendritic growth across tracks on a PCB

 

Dendrite growth across resist

Dendrite growth across resist

 

The rate of electromigration increases with temperature and has four prerequisites – a mobile metal; a voltage gradient; a continuous film of moisture; soluble ions:

The move to finer pitch components, and consequently reduced spacings, makes electromigration more likely, because the voltage gradients between conductors increase and it becomes more difficult to eliminate the minute amounts of ionic contaminants which are sufficient to cause problems.

Three main methods have been used to reduce the electromigration problem:

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Author: Martin Tarr


 

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